Not Applicable
Not Applicable
1. Technical Field
This invention pertains to oil and gas exploration and production in general, and in particular, to a method for the assembly and deployment of an extendable draft platform for deep water offshore exploration and production of oil and gas.
2. Related Art
Development of deep water offshore oil and gas fields such as are found in, e.g., the Gulf of Mexico and the North Sea present substantial challenges to the industry. Early production schedule requirements favor inshore integration and commissioning and a year-round deployment capability, even in winter months. Moreover, the ability to use so-called xe2x80x9cdry treesxe2x80x9d and steel catenary risers (xe2x80x9cSCRsxe2x80x9d) requires that the motion of the deployed structures be relatively small, even in rough seas.
In response to these challenges, a number of different types of tethered, floating platforms have been proposed for use in offshore oil and gas exploration and production, such as are described in U.S. Pat. Nos. 6,024,040xe2x80x94Thomas; 6,196,767xe2x80x94Thomas; and U.S. application Ser. No. 09/686,535 now abandoned; all of which are commonly owned by the assignee hereof, and the teachings of which are incorporated herein by this reference. While these platforms have to some extent been responsive in meeting some of the above challenges, they still leave areas in which improvement is needed.
For example, to obtain low motion characteristics in a floating facility, e.g., those in which dry-tree well completions and/or the use of SCRs is contemplated, it is necessary to use a deep draft vessel and/or heave-suppression devices. Because of these requirements for a large draft, current methods for deploying oil-industry-related platforms that have favorable motion characteristics in deep water are typically effected sequentially, and require offshore heavy lift crane vessels (xe2x80x9cHLCVsxe2x80x9d) or xe2x80x9cfloat-overxe2x80x9d deck-mating techniques that are inherently risky. These methods also require that the weather conditions be generally favorable, and a substantial amount of time can be lost waiting for an opportunity to complete the commissioning and mating operations. Therefore, more rapid, cost-effective and less risky methods are needed for the provisioning and deployment of an offshore platform.
In accordance with the present invention, a method for the assembly and deployment of an extendable draft platform (xe2x80x9cEDPxe2x80x9d) to a deep water offshore location is presented that is rapid, cost-effective, and less risky. The method combines the advantages of both xe2x80x9cjack-upxe2x80x9d and xe2x80x9csemisubmersiblexe2x80x9d platforms, permits many of the deployment steps to be effected in parallel, rather than sequentially, and is self-contained, i.e., eliminates the need for auxiliary equipment and vessels such as HLCVs or float-over deck-mating techniques.
The novel method comprises providing a buoyant equipment deck having a plurality of leg openings through it and a plurality of chain winches mounted thereon. Each winch has the proximal end of a mooring chain wound onto it. In one exemplary preferred embodiment, the equipment deck of the EDP comprises a triangular, barge-like hull structure having a triangular central opening, with the leg openings being located inboard of the apices of the hull.
An adjustably buoyant pontoon is also provided that, in one exemplary preferred embodiment, comprises a deep, triangular-shaped hull having a central opening and approximately the same peripheral outline as the equipment deck. The pontoon has a plurality of fairleads disposed around its lateral periphery and a plurality of columns, or legs, extending upwardly therefrom. Each leg comprises an adjustably buoyant float, which may be circular or polygonal in cross-section. Optionally, each leg may include a lower portion comprising an elongated, open truss, which, in one possible embodiment, may have a triangular cross-section.
The deck is placed on the pontoon such that the pontoon supports the deck and each of the legs extends up through a respective one of the leg openings in the deck. The distal ends of the mooring chains are reeved through corresponding ones of the fairleads on the pontoon and then connected back to the deck. The chains are then wound in with the winches so that the deck is pulled down tightly onto and closely coupled with the pontoon. This tightly coupled xe2x80x9cpiggybackxe2x80x9d configuration enables the deck and pontoon to be moved quickly and securely as a unit over a surface of a body of water, e.g., by towing, to a first offshore location, without the need for any additional sea fastenings between the deck and pontoon.
A plurality of pawls are provided on each of the leg floats. These pawls, which may comprise compression pawls or shear pawls, are movable between a retracted position, which permits relative vertical movement between the legs and the deck, and an extended position, in which they can be engaged with the deck to prevent relative vertical movement between the legs and the deck. This arrangement, in combination with the mooring chains and winches, enables the pontoon to be submerged to a xe2x80x9cworkingxe2x80x9d depth below, and the deck to be raised to a xe2x80x9cworkingxe2x80x9d height above, the surface of the water at the first offshore location, with the following procedure.
The buoyancy of the pontoon and the leg floats is reduced, e.g., by ballasting them with sea water, such that they begin to sink. The chains are simultaneously wound out with the winches while maintaining a positive tension in them, so that the deck is floated onto the surface of the water while the pontoon and legs sink below the surface to selected depths. The pawls on the leg floats are then moved to the extended position and the chains are wound back in with the winches such that the pontoon and the legs are pulled back up toward the deck, until the pawls engage the deck, thereby preventing further relative movement between the legs and the deck. The buoyancy of at least one of the pontoon and the leg floats is then increased such that the deck is thereby raised to a selected working height above the surface of the water. In one exemplary preferred embodiment, means are provided between the deck and the pawls for distributing the load imposed by the deck on respective ones of the leg floats uniformly around a circumference of the floats when the pawls engage the deck.
After the EDP has been extended to the above xe2x80x9cworkingxe2x80x9d configuration, it is towed to an offshore well site having a plurality of mooring lines anchored to the seabed around it. There, the deck is connected to the leg floats in a more permanent manner, e.g., by welding, to prevent relative movement between the deck and the leg floats independently of the tension in the mooring chains. The distal ends of the mooring chains may then be disconnected from the deck and connected to the ends of respective ones of the mooring lines such that the platform can be maintained in a selected working position over the well site with the winches.
As may be seen from the above, the platform can be provided in a retracted, shallow draft configuration suitable for assembly, outfitting and towing in shallow coastal waters, and can then be easily and quickly reconfigured in deeper offshore waters to a very deep-draft, low-motion platform using simple, onboard ballasting and mooring equipment, thereby resulting in a rapid, cost-effective, and less risky deployment, without the need for auxiliary vessels and equipment.
A better understanding of the above and many other features and advantages of the present invention may be obtained from a consideration of the detailed description thereof below, particularly if such consideration is made in conjunction with the appended drawings.